1. Field of the Invention
The present invention relates to an amplitude setting circuit.
2. Description of the Related Art
Through communication networks such as LANs in offices and vehicle-mounted networks, digital signals are transmitted between apparatuses in the form of signals of various formats. Communication networks are beginning to be used to connect various digital apparatuses other than computers as well as being used to connect computers and their peripherals. An example thereof is vehicle-mounted networks and, for example, a MOST (Media Oriented Systems Transport) system has been proposed as a standard for vehicle-mounted networks. In the MOST system, a ring-like vehicle-mounted network is configured, and various apparatuses such as a car navigation system, a CD/DVD player, a speaker, a display, a telephone are connected to the network. The vehicle-mounted network is used, for example, in a way that the CD/DVD player transmits a reproduced digital signal to the speaker via the vehicle-mounted network and that the speaker converts the digital signal into voice and outputs.
As a digital signal transmission method in a communication network, there are a base band method in which a digital signal is transmitted as it is and a broad band method in which an analog signal obtained by modulating a carrier wave with a digital signal is transmitted. An amplitude shift keying method as a broad band method will be described in detail below.
FIG. 19 is a diagram illustrating the configuration of a conventional ASK modulation circuit. The ASK modulation circuit of FIG. 19 has transmit data D, that is serial digital data, inputted thereto and generates an ASK modulated signal S whose amplitude varies in response to changes in the bit value of the transmit data D over time and outputs to a network.
With reference to the waveforms of main signals of the conventional ASK modulation circuit of FIG. 20 as needed, the configuration of the ASK modulation circuit of FIG. 19 will be described.
A reference clock generator 10 generates a clock signal CL of a frequency proportional to the bit rate of the transmit data D. Let r be the bit rate (bps) of the transmit data D, then the frequency of the clock signal CL is expressed as n (natural number)×r (Hz).
Amplifiers 12, 14 each have the clock signal CL of a rectangular waveform inputted thereto and have their output amplitude level decided by a predetermined gain. For example, the amplifiers 12, 14 produce respective clock signals CL1, CL2 of a rectangular waveform that swing to their peak and bottom with ground potential (zero level) as their reference (see (a), (b) of FIG. 20). Note that the clock signals CL1, CL2 are set to have amplitude levels different from each other.
A switching controller 20 latches the bit value of the transmit data D synchronously with the clock signal CL and depending on the latched bit value, generates a control signal SW to control the on/off of switches 16, 18 (see (c) of FIG. 20).
The switches 16, 18 switch on/off complementarily according to the control signal SW supplied from the switching controller 20. For example, when the control signal SW is at a High (H) level, the switch 16 is off and the switch 18 is on. When the control signal SW is at a Low (L) level, the switch 16 is on and the switch 18 is off. A combined signal M of the outputs of the switches 16, 18 (see (d) of FIG. 20) is input to an LPF 22.
The LPF 22 removes high frequency components from the combined signal of the outputs of the switches 16, 18 and produces the ASK modulated signal S of a smooth sine waveform (see (e) of FIG. 20).
Such a conventional ASK modulation circuit is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2001-119442.
Fixed gain amplifiers like the amplifiers 12, 14 of FIG. 19 and variable gain amplifiers such as an electronic volume and an attenuator are amplitude setting circuits that set the amplitude level of their output signal corresponding to an input signal. These conventional amplitude setting circuits have the problem that their amplifier circuit elements have a temperature characteristic and thus their gains vary with temperature. Therefore, the output signal having a set amplitude level varies in amplitude with temperature.